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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 04 Jul 2010 (Sunday) 13:57
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M13 in Hercules

 
DonR
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Jul 04, 2010 13:57 |  #1

I spent some time early last night imaging globular cluster M13, and waiting for full darkness to shoot M101. This is 30 three minute frames at 1000mm and ISO 400 from my Digital Rebel XT, plus darks, flats, dark flats and bias frames, processed in Iris and finished in Photoshop.

About 11:30 PM I switched over to galaxy M101, planning to get a few shots before the moon got up too high, but high, thin clouds came in. I will try again tonight if the weather forecast holds.

Don


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Adrena1in
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Jul 04, 2010 15:39 |  #2

Very nice, I do love M13. But am I seeing some sort of smearing of all the stars around the edge? They all look a bit smeared away from centre.


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DonR
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Jul 04, 2010 16:03 |  #3

Thanks, Adrena1in.

Yes, that's coma, a fact of life with newtonian reflectors at f/5. There are coma correctors available, but they are not easily used on some telescopes, including mine. I always notice it, but it doesn't really bother me.

Don




  
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astrostu
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Jul 04, 2010 21:15 |  #4

Yeah, I noticed that too. Not that big of an issue, and you can always crop for the smaller stuff like this.

I am curious, though, as to why you did 30 3-minute frames instead of something longer. Were you having heat or tracking issues?


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DonR
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Jul 05, 2010 09:48 |  #5

astrostu wrote in post #10477820 (external link)
I am curious, though, as to why you did 30 3-minute frames instead of something longer. Were you having heat or tracking issues?

The problems on this night were light pollution, sky transparency and time.

Light pollution has always been a problem here, and it's getting worse. Sky transparency is often a problem here in the summer. These two factors combined to make three minutes at ISO 400 about the most exposure I could use. I have attached a JPEG of a single raw frame to demonstrate.

I could have dropped the ISO to 200 and doubled the exposure time, but I was planning to shoot M101 later in the evening, and I wanted more frames than I could get in the available time last night with six minute exposures. In theory, stacking half as many frames at twice the exposure time yields the same SNR, but in my experience and in the conditions under which I image the minimum number of frames needed to produce an optimum stacked image is more than 15.

Since M13 is relatively bright with high contrast, three minutes is enough exposure time. With a longer exposure I would have captured a few more of the faint stars surrounding the cluster, but I would have risked blowing out the core.

Don


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DonR
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Jul 05, 2010 10:00 |  #6

Here's a closer crop, getting rid of most of the coma and showing more detail and color in the core.

There's still some coma apparent at the top and left side, due to the fact that the subject was a little off-center in the field.

I did image M101 last night, but again the sky transparency wasn't good. If I can make a decent image out of it I will post it.

Don


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David ­ Ransley
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Jul 05, 2010 19:40 |  #7

Nice - and to think that what we see here is about 145 light years across and 25000 light years way from us if I am correct. This is a view of the past :-)


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NovaTJ
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Jul 05, 2010 19:50 as a reply to  @ David Ransley's post |  #8

Very nice effort DonR. I too tried M13 two nights ago and gave up with frustration over light pollution. I was trying to image at iso 800 and was totally blown out. Even post processing couldn't help enough. I like your image even with the coma. My 6 inch F5 newt is that way too. I usually just crop it and go with the results.

Greg


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DonR
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Jul 06, 2010 12:26 |  #9

Thanks David and Greg.

Globular clusters are beautiful objects, especially the bright ones like M13. And if you're imaging under light pollution, poor transparency and/or moonlight, they are sometimes the only suitable DSO subjects up there. I have gotten decent images of bright globular clusters even when the moon is full and high in the sky. You can forget about galaxies and most nebulae under these conditions.

Don't be afraid to drop the ISO, Greg, when the sky glow is severe. Signal is photons, and dropping the ISO doesn't reduce the number of photons captured. Dropping the ISO under these conditions makes the post-processing easier, since the sky background is darker.

Don




  
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ecce_lex
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Jul 07, 2010 02:36 |  #10

Hi there,

Very sweet result on THE cluster, congrats :)

Was wondering what instrument you use cause I just bought a f/5 reflector (newt) @ 750mm focal length but haven't had the chance to shoot anything but the moon.

Very nicely resolved stars, lotsa color - nice image. Crop blew me away.

Cheers


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NovaTJ
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Jul 07, 2010 05:41 as a reply to  @ ecce_lex's post |  #11

Thanks Don, I will try again at a lower ISO when this heat wave subsides...


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DonR
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Jul 07, 2010 16:21 |  #12

ecce_lex wrote in post #10490977 (external link)
Hi there,

Very sweet result on THE cluster, congrats :)

Was wondering what instrument you use cause I just bought a f/5 reflector (newt) @ 750mm focal length but haven't had the chance to shoot anything but the moon.

Very nicely resolved stars, lotsa color - nice image. Crop blew me away.

Cheers

Thanks!

My newt is an 8" (203mm) f/4.9, so focal length is ~1000mm.

Your 6" f/5 should do a good job too. Collimation, focus and tracking/guiding are the keys to getting good star resolution, along with a factor you can't control - the seeing.

The keys to getting good star color are not over-exposing, and of course, image processing. Since I started using Iris software for processing, this has become easier for me. Iris doesn't add color that's not there, but helps bring out the color that is there. The same could be done in Photoshop after calibrating and registering in DSS, for example, but it would be much more difficult.

Don




  
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ecce_lex
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Jul 08, 2010 02:14 as a reply to  @ DonR's post |  #13

I've used IRIS when I started but gave it up for registax and dss: It's the linux of astrophoto software, pretty unfriendly and lotsa command lines, no?

wouldn't mind giving it a try again - do you have a routine? maybe you can give me a list of commands :)

cheers


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DonR
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Jul 08, 2010 13:35 |  #14

ecce_lex wrote in post #10497348 (external link)
I've used IRIS when I started but gave it up for registax and dss: It's the linux of astrophoto software, pretty unfriendly and lotsa command lines, no?

wouldn't mind giving it a try again - do you have a routine? maybe you can give me a list of commands :)

cheers

Yes, Iris is not especially user friendly, but that's what makes it so powerful - the user has complete control of every aspect of the image processing.

When I first looked at Iris, I gave up after a few hours for the same reason you cite. Later I found Jim Solomon's Astrophotography Cookbook (external link) on the web, and the first time I followed his steps, I succeeded in producing my best astro image up to that time. I highly recommend that you have a look at it.

Later, I developed a set of scripts to use that essentially provided some automation to the process in Jim Solomon's cookbook. The scripts are simply a listing of the various Iris commands I use, in sequential order. I made a web page describing the scripts in some detail, and including a link to download the scripts:

http://dandjreed.homed​ns.org/iris/iris.html (external link)

I recommend first using Jim Solomon's cookbook to gain some familiarity with Iris and the image processing steps, then try my scripts, or study them and make your own scripts. It's still not as easy or as fast as DSS, but in my experience the results are usually superior.

There are five separate scripts, and they break down Jim's process into five steps:

- creating the master flat image
- creating the master dark image
- calibrating the light images
- registering the calibrated images
- stacking the registered images

The result from the fifth step will need some post-processing in Photoshop, just as the result from DSS does.

The first four steps seldom if ever need tweaking. The fifth step just about always produces a decent result, at least as good as the result from DSS. Occasionally, I find it necessary to do some manual work in Iris using the intermediate images produced by the fifth script in order to tweak the result before taking it into Photoshop for final processing. My web page goes into some detail on that.

Good luck,

Don




  
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ecce_lex
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Jul 09, 2010 02:36 |  #15

Whoa. Thanks a lot Don :)

It's about time I got back to IRIS... I'm definetly having a look, I'll let ya know what comes of it.

Thanks again


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M13 in Hercules
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